Hitherto, as an insulation circuit board for an electronic apparatus which is subjected to a severe temperature environment such as a vehicle-mounting environment or the like, a ceramic board having excellent heat radiation performance and heat resistance is a main stream. However, in association with a demand for low costs of the electronic apparatus, a shift to a resin board in which a high radiation insulation resin is used for an insulation layer is increasing year by year.
As shown in FIG. 2, the insulation circuit board by the resin insulation layer (hereinbelow, referred to as an insulation circuit board) is constructed in such a manner that a metal foil such as a copper foil or the like which was coarse-processed is adhered to one or both of the surfaces of a metal base substrate 1 such as aluminum, copper, iron, or the like through an insulation layer 2 whose thermal conductivity has been improved such as a high thermal conductive resin or the like having excellent heat radiation performance, and thereafter, wiring patterns 3 are formed by a chemical etching or the like so as to be away from each other by a distance R1 between the wiring patterns. For example, in the case of a power semiconductor device or inverter module, a plurality of semiconductor elements and passive elements are arranged on the wiring patterns 3 on the insulation circuit board. However, since a high power switching element such as an insulated-gate bipolar transistor (IGBT) or the like is used as a power semiconductor device for electric power conversion or an inverter module in recent years, there is a problem of an excessive heat generation. At present, in order to efficiently propagate the generated heat to the metal base substrate 1, an inorganic filler is filled into the resin of the insulation layer 2 at a high density or a thickness of insulation layer 2 is made thin, thereby improving the thermal conductivity of the insulation layer 2. As for the former method of filling the filler into the resin of the insulation layer 2 at the high density, an insulation circuit board in which an alumina filler of 70 Vol % or more was mixed into the insulation resin which is in contact with the metal base substrate 1 has also been developed, and high thermal conductivity which reaches 10 W/mk has been realized. As for the latter method of making the thickness of insulation layer 2 thin, an insulation circuit board using the insulation layer 2 having a layer thickness of about 200 μmt or having the smallest layer thickness of about 100 μmt has been developed. However, if a high voltage associated with the realization of the recent high power is always applied to such an insulation circuit board, such a problem that a dielectric breakdown occurs for a short time can occur.
The dielectric breakdown mentioned above will now be described by using FIG. 3. As shown in FIG. 3, in the wiring pattern 3 to which the high voltage is applied, the generation of partial discharging 4 that is caused by electric field concentration at end sections of the wiring pattern 3 is a main cause. Particularly, in the case of an AC voltage in which a polarity of the applied voltage changes continuously, the discharge occurs continuously, so that the insulation layer 2 is deteriorated, a dendritic discharge deterioration trace called a tree 5 is formed, and a dielectric breakdown occurs soon. Also in the case where the wiring patterns 3 are closely formed due to the high density of mounted parts, a dendritic discharge path (creeping discharge 6) is formed along the surface of an insulation material by the partial discharging 4 and a dielectric breakdown occurs. At present, the wiring pattern 3 which is formed by the chemical etching or the like from a viewpoint of priority of costs generally has such a shape that a side surface portion of the wiring pattern 3 becomes an arc. It has such a shape that the side surface portion of the wiring pattern 3, in more detail, the side surface portion of the wiring pattern 3 which is in contact with a creeping portion of the insulation layer 2 is sharply pointed. There is consequently such a problem that an electric field is concentrated on the end sections of the wiring pattern 3 and the partial discharging 4 occurs easily at this portion as a starting point. In order to lighten the electric field concentration at the end sections of the wiring pattern 3, it is effective to round the sharp shape of the end section of the wiring pattern 3. In [Patent Literature 1], by forming the wiring pattern 3 into a concave portion 7 of the insulation layer 2 in which peripheral end sections are curved as shown in FIG. 4, the end section of the wiring pattern 3 has a round shape. Further, in [Patent Literature 2], a method of fusing and smoothing the end section of the wiring pattern 3 by a pre-discharge, a laser, or the like has been proposed.